1. Field of the Invention
This invention relates to printing devices and more particularly to a method and means for flushing air from a printing head prior to initial use when the printing head is equipped with a removable ink supply tank.
2. Prior Art
In printing equipment of the type utilizing liquid ink, a printer head which is mounted for movement adjacent a recording carrier, such as paper, may be supplied from a liquid ink supply which includes a supply flask containing the printing ink. Such equipment may, for example, comprise a mosaic printer head having a plurality of piezoelectrically operated printing ink jet nozzles arranged at an operating face of the printer head. Such printing heads are automatically supplied with printing ink by means of the contraction process of the individual printing jets operating in the print head.
In order for such ink jet printers to operate properly in accordance with the physical principles utilized in their operation, it is imperative that all air be eliminated from the ink supply system. As a step in the elimination of air, it has been suggested to provide a device within the ink supply tank which will make it possible to increase the tank pressure and, thus, the pressure on the ink supply for short periods of time in order to force ink through the printer head so as to flush the printer head with the intention of expelling all air from within the printer head.
However, it is common practice to form the printer head from a casting resin. Such casting resins frequently have physical characteristics such that they are only poorly wetted with ink even when the printing head is filled with ink. Thus, particularly when using casting resins, and particularly at the beginning of operation of an individual print head it is possible that the ink supply channels will not be ink wetted even though the individual channels will be otherwise filled with ink. In this instance, small air bubbles will remain within the channels, adhere to the channels walls. The small air bubbles cannot be flushed even if the printing head is subjected to continuous flushing by printing ink. Such enclosed air bubbles can act to prevent proper expulsion of the ink drops from the printer head particularly since they are able to act in a manner which absorbs the pressure surges generated by the piezo transducers. Thus, the printing head will not properly function until all of the air bubbles have been dissolved in the printing ink which may take some considerable period of time. Therefore, the printing head is not immediately available for printing.
This problem has been previously considered, and as a attempted solution thereto, it has been suggested to eliminate such de-aerating problems when exchanging printing heads by filling the printing heads with a neutral colorless liquid during storage and transport. See for example German Offenlegungsschrift No. 2,317,911. According to this method, the printing head is placed in a special transport container during transport, as for example, from the factory to the print head utilizer. After removal of the print head from the transport container and connection of the print head to the printing device, the print head is activated for a period of time. The neutral liquid which is ejected from the print head during this initial activation period is diverted from the working phase of the print head and is absorbed in some manner until such time at the stream emitted from the print head becomes the stream of printing ink from the ink supply. While this type of approach reduces the aeration problem, it is extremely expensive and requires a multiplicity of specific technical devices and makes transportation and storage cumbersome. It would therefore be an improvement in the art to provide a method and apparatus for properly de-aerating printing heads, particularly of the ink jet type which proceeds in a simple and inexpensive manner to eliminate the problem of small air bubble entrapment within the printing head, particularly at the onset of initial use of a printing head.